Conventional suspension cultures for the production of such proteins as antibodies and hormones in vitro are limited in their applicability to large scale use by reason of the low cell densities (10.sup.6 cells/ml) which are possible. Relatively large and complex fermentors are required for commercial production with consequent heavy capital expenditures, and even then the concentration of desired product is quite low. Further, most cell lines grow best in serum supplemented medium, but serum proteins greatly complicate the downstream purification of desired protein products from supplemented media.
Alternative processing has therefore been suggested and emphasis has been laid upon microencapsulation to overcome the problems of low cell density and downstream processing. In this process, which has been extensively described in the open scientific literature and in numerous patents, living cells are entrapped within a semipermeable polymer membrane. The living cells are mixed with sodium alginate and extruded into calcium chloride to form calcium alginate gel droplets. The gelled droplets are then reacted with a polyamino acid such as poly-l-lysine (PLL) to form a semipermeable capsule membrane. The interior is then liquified by incubating the capsules in sodium citrate. The encapsulated cells, such as hybridoma cells are then transferred to a suitable medium and incubated for two to three weeks. The concentration and purity of the product is about 100 fold higher than can be achieved by conventional cell suspension cultures. Attention is directed to:
U.S. Pat. Nos. 3,157,631 PA1 U.S. Pat. Nos. 3,780,195 PA1 U.S. Pat. Nos. 4,251,387 PA1 U.S. Pat. Nos. 4,255,411 PA1 U.S. Pat. Nos. 4,257,884 PA1 U.S. Pat. Nos. 4,322,311 PA1 U.S. Pat. Nos. 4,324,683 PA1 U.S. Pat. Nos. 4,352,883 PA1 U.S. Pat. Nos. 4,386,895 PA1 U.S. Pat. Nos. 4,389,419 PA1 U.S. Pat. Nos. 4,391,909 PA1 U.S. Pat. Nos. 4,407,957 PA1 U.S. Pat. Nos. 4,409,331 PA1 U.S. Pat. Nos. 4,487,758 PA1 U.S. Pat. Nos. 4,495,288 PA1 U.S. Pat. Nos. 4,582,799
which are considered to be generally relevant to the process of microencapsulation described hereinabove. It has, however, been established (Posillico, Biotechnology, 4, 1986) that cells entrapped within the single membrane capsule of the prior art tend to grow preferentially near the interior surface of the microcapsule. By the end of the culture period the cells cover about one half to three quarters of the interior capsule surface but less than one third of the total capsule volume is occupied by the cells: the other two thirds being occupied by calcium and sodium alginate polymer. It is apparent, therefore, that only part of each microcapsule is available for cell growth as the alginate (Mv 10.sup.5 to 10.sup.6) is entrapped within the capsule which has a membrane molecular weight cut-off of about 60.times.10.sup.3 to 80.times.10.sup.3 in order to entrap the antibody. Further, when the capsules are ruptured to recover the product the entrapped alginate has to be separated out from the suspension of ruptured cells and proteins. There is, therefore, a need for an improved process for significantly increasing the intracapsular cell density and for removing the alginate core prior to incubation of the microcapsules.